Sodium ion batteries are very similar in many ways to lithium ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy and they both charge and discharge via a similar reaction mechanism. When a sodium-ion battery (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate and migrate towards the anode whilst charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction. Once a circuit is complete electrons pass back from the anode to the cathode and the Na+ (or LI+) ions travel back to the cathode.
Lithium-ion battery technology has been utilised in many applications, and is used in portable devices extensively; however lithium is not a hugely abundant material and is expensive to use in large scale applications. Sodium-ion technology is still a new technology but the high abundance of sodium on the earth, and a significantly lower cost of sodium compared to lithium give sodium ion an advantage over lithium ion technologies. Researchers predict that sodium ion will provide a cheaper and more durable way to store energy in the future, especially for large scale applications such as grid level energy storage.
US 2012/0227252 A1 describes the preparation of lithium transition metal silicates, and in particular the preparation of a silicate cathode for a lithium ion battery comprising: preparing an Olivine structure having a flake-like structure; carbon coating the Olivine structure; and shaping the Olivine structure for use as part of a cathode.
US 2013/0052544 A1 (2010) teaches a cathode material which contains a lithium transition metal silicate of small particle size and low crystallinity. The material is described to be a useful cathode active material in a non-aqueous electrolyte secondary battery, capable of undergoing a charge-discharge reaction at room temperature.
However, not all lithium transition metal silicates are found to be effective in battery applications. A literature review: “Silicate Cathodes for Lithium Batteries: Alternatives To Phosphates?” by Bruce et al in J. Mater. Chem., 2011, 21, 9811, highlights the difficulties encountered when Li2MSiO4 compounds are used. For example, R. Dominko et al., in J. Power Sources, Vol. 174, Issue 2, 6 Dec. 2007, pp 457-461, report that lithium extraction from Li2MnSiO4 during initial charging appears to cause significant structural changes so that the resulting material is only able to reversibly exchange limited amount of lithium.
Li2MetalSiO4 compounds also generally have low rate capabilities compared with their phosphate analogues, and as reported by Bruce et al. in Chem. Commun., 2007, 4890-4892, the capacity to extract lithium is very low from all cobalt polymorphs. Although they also report that this could be improved when these materials were coated with carbon, they found that this was very difficult to achieve because when the material was fired to the carbonisation temperatures, it reduced to lithium silicate and cobalt metal. The only polymorph which they managed to carbon coat, had a reversible capacity of only 60 mAhg−1.
Further literature for example by Nyten et al in Electrochemistry Communications 7 (2005) 156-160, reports the electrochemical performance of Li2FeSiO4, Li2FeGeO4, and Yang et al in Electrochemical and Solid State Letters, 11 (5) A60-63 (2008) and in Electrochemical and Solid State Letters, 12 (7) A136-39 (2009), report that the electrochemical performance of Li2FeSiO4 can be Improved by the addition of a carbon coating.
In a different approach, WO 2010/066439 A2, describes alkali metal doped phosphate materials which are reported to be electrochemically active and suitable electrode materials for use in primary and secondary batteries. The compounds described contain between 60 and 90 Mol % phosphate ions (PO43−), and although relatively small amounts (up to a maximum of 31 Mol %) of the phosphate ions may be substituted with one or more anions including silicate ions (SiO44−), this prior art warns against substituting more than about 30 Mol % of the PO43− because the resulting compounds are unstable.
Sodium ion analogues Na2MgSiO4 and Na2ZnSiO4 are reported in Solid State Ionics 7 (1982) 157-164; Solid State Ionics 18 &19 (1986) 577-581 and Mat. Res. Bull., (1989), Vol. 24, pages 833-843, to be useful ionic conductors. However, although conductivity tests have been performed, neither magnesium nor zinc in these materials is redox active and consequently, the materials are not capable of sodium removal on first charge or of being useful in a sodium ion battery application.
Similarly, WO2014/050086 (Toyota) teaches NaFeSi2O6 materials in which the oxidation state of the iron is +3. Consequently, this compound is not redox active upon oxidation, and is not capable of sodium removal on first charge or of being useful in a sodium-ion battery application.
WO2012/004839A1 (Mitsubishi) discloses some 43 sodium-oxide composite materials for use in the positive electrode of a secondary battery, including Na2MnSiO4. However, the present Applicant has found that this manganese material exhibits low reversible capacity in a sodium-ion cell; consequently, this compound is specifically excluded from the active materials used in the electrode of the present invention.
U.S. Pat. No. 6,872,492 B2 (Valence) describes a material for a sodium ion cathode with the formula of AaMb(XY4)cZd, where (XY4) is defined as selected from the group consisting of X′O4-x, Y′x, X′O4−y, Y′2y, X″S4, and mixtures thereof, where X′ is selected from the group consisting of P, As, Sb, Si, Ge, S and mixtures thereof; X″ is selected from the group consisting of P, As, Sb, Si, Ge and mixtures thereof; Y′ is selected from the group consisting of halogen, S, N, and mixtures thereof; 0≤x<3; and 0<y≤2; and 0<c≤3. This patent encompasses a very large number of materials, however, there are no specific examples relating to the silicate containing materials used in the present invention, moreover, there is no electrochemical data or any other evidence which, in view of the capricious nature of the lithium analogues, is needed to show, or to assist the skilled person to predict, which if any of the SiO4-containing materials have suitable electrochemical characteristics for use in an electrode or as being able to perform in a sodium-ion battery application.
US2003/170542A1 (Valence) broadly discloses a large number of electrode active compounds comprising the general structure: Aa+xMbP1−xSixO4, in which x may be in the range 0≤x≤1. As with the Valence patent document discussed above, none of the compounds used in the present invention are disclosed in an individualised form in this prior art, nor is any evidence given in this prior art regarding the ability of such materials to exhibit favourable electrochemical characteristics, or any clue that would assist the skilled person to predict that the compounds of the present invention are capable of sodium removal on first charge.
Finally, U.S. Pat. No. 4,166,159A (Pober) discloses Na1+xZr2SixP3−xO12 materials for use as solid electrolytes in sodium (but not sodium-ion) batteries. Such prior art sodium batteries do not rely on the intercalation of sodium ions and are not capable of reversible charge-discharge cycling; i.e. they are not rechargeable. There is no teaching in this prior art that such materials are useful in electrodes for sodium-ion batteries or are capable of producing electrodes which undergo sodium removal on first charge or which are capable of multiple chare-discharge cycles.
In conclusion, none of the above literature references provide any basis on which to judge which sodium metal silicates are capable of reversibly intercalating sodium ions during repeated charge-discharge cycles. Specifically, none of the prior art discussed above teaches which sodium-containing transition metal silicates are able to undergo sodium removal on first charge, or, which would be suitable active materials for use in electrodes of the present invention, in sodium-ion battery applications, particularly in cathode electrodes.